In closed-loop Multiple Input Multiple Output (MIMO) system, receiver needs to feed channel information back to transmitter for the transmitting data signal during communication.
Considering a MIMO system with Nt transmitting antennas and Nr receiving antennas, Mt denotes the number of space multiplex data flows to be sent, vector x of Mt×1 denotes the signal carried by the data flows, the precoding matrix W is a Nt×Mt dimensional matrix, which transforms vector x into Nt×1 dimensional vector z:z=Wx Vector z is actual signal transmitted by transmitting antenna. The signal received by receiving antenna is denoted as:r=HWx+n Wherein, H denotes channel matrix, n denotes Gaussian white noise vector.
If feedback overhead is not considered, the optimal choice of W is right singular vector of Matrix H. However, the cost of feedback of these singular vectors is great. Therefore, a solution of closed-loop MIMO precoding with limited feedback is proposed. A group of precoding matrixes or vectors are structured for each transmitting antenna dimension, and this group of precoding matrixes or vectors (called as matrix below) are called as “codebook”. Both base station and mobile station are enabled to know this codebook, which is denoted as P={P1, . . . , PL}, in which each matrix is called as a “code word”. If L=2q denotes the size of codebook, q is the required bit number for indexing the codebook. Take an 4×2 MIMO system as an example, for a codebook with size L=64, only 6 bits information is needed to feed back for the transmitter to select precoding matrix. After the codebook of a MIMO system is determined, the receiver detects the channel and selects the optimal code word (precoding matrix) for the present moment, and then the index of the code word is fed back to the transmitter. The MIMO system may save a plenty of feedback overhead by means of such limited codebook index feedback.
The MIMO system may also use a limited feedback solution of feeding back quantized channel matrix codebook index, that is: quantized channel matrix codebook is structured in advance and stored at the transmitter and receiver; the receiver detects the channel and selects the optimal code word for the present moment from the quantized channel matrix codebook and feeds back the index of the code word; the transmitter determines present channel transmission matrix according to the code word corresponding to the index, and then calculates the optimal precoding matrix for data transmission according to present channel transmission matrix.
In 3GPP RAN1 discussion, it has been agreed about the feedback of channel information in LTE-A (Long Term Evolution system-Advanced) system that: two code words may be respectively selected from two separate codebooks and reported to the base station, when the mobile terminal feeds the precoding matrix back to the base station. In the two code words, one is used for representing wideband and/or long-term channel properties; the other is used for representing frequency-selective (such as a sub-band) and/or short-term channel properties. The base station obtains the precoding matrix according to the product of the two code words. The user terminal may determine the code word for representing wideband and/or long-term channel properties according to capacity maximization principle or the principle of minimum distance of eigenvector of channel correlation matrix, and then determine the code word for representing frequency selective (such as a sub-band) and/or short-term channel properties according to real-time channel condition.
In practice, cross-polarized linear array (CLA) is a typical manner of antenna configuration. Antenna polarization represents the direction of electric field strength formed during antenna radiation. The electrical wave is called as vertically polarized wave if the direction of electric field strength is perpendicular to ground; the electrical wave is called as horizontal polarized wave if the direction of electric field strength is parallel to ground. Cross-polarized antenna is one kind of dual-polarized antenna.